Three years ago, the Red Sox fell about $5 million short of the White Sox's six-year, $68 million winning bid for Abreu, a slugging first baseman from Cuba.

Abreu went on to be crowned American League Rookie of the Year in 2014, following in the footsteps of successful Cuban defectors Yoenis Cespedes and Yasiel Puig, and Boston resolved it would not be outbid for the next touted Cuban player.

Sure enough, in August 2014, the Red Sox signed Castillo for seven years and $72.5 million, the richest contract ever for an international player.

On Monday, Castillo became one of the highest-paid players ever to be removed from a 40-man roster, the Red Sox outrighting him to Triple-A Pawtucket after giving him only eight at-bats this season. Castillo was placed on waivers Saturday, but as expected, he went unclaimed.

The Red Sox had grown frustrated with Castillo's lack of improvement at the plate, specifically his inability to catch up to major league fastballs. He went into spring training as the leading candidate to win the left-field job but was outplayed by super-utility man Brock Holt. And despite opening the season on Boston's bench, he was swiftly optioned to Triple-A, where he was batting only .245 with eight doubles, one homer and a .624 OPS in 38 games.

When Holt went on the disabled list last month with a concussion, the Red Sox bypassed Castillo in favor of catcher-turned-outfielder Blake Swihart. When Swihart sprained his left ankle three weeks ago, lefty-mashing veteran Chris Young began playing against right-handed pitchers, too. Castillo got called up, but Farrell used him almost exclusively as a pinch runner until he was replaced on the roster last weekend by outfielder Ryan LaMarre.

Holt was scheduled to begin a minor league rehab assignment by playing five innings Monday night for Triple-A Pawtucket. He's expected to play seven innings Tuesday, take a day off Wednesday and play nine innings Thursday before being re-evaluated prior to the opener of a six-game road trip Friday night in Texas.

Being removed from the 40-man roster doesn't preclude Castillo from being called up again. But it's an indication that he doesn't factor into the Red Sox's plans any time soon.

"It's pretty simple: It's an offensive production aspect of it," Red Sox president of baseball operations Dave Dombrowski said. "He's a good defensive outfielder. He's got a good arm. He runs well. But he hasn't really hit on a consistent basis. So, that's what it really comes down to."

Castillo showed flashes of being a competent big league hitter during the second half of last season, batting .321 with six doubles, two triples, four homers and an .838 OPS in 38 games from July 27 through Sept. 13. But while teams changed the way they pitched to him, Castillo was unable to make the counter-adjustments to carry over his success.

"Whether it's approach or swing mechanics or a combination of the two, that's what was clearly at play here," Farrell said. "While there was some stretch of production there, the league is pretty smart and information gets out on everybody."

Dombrowski has no ties to Castillo, who was signed by former Red Sox general manager Ben Cherington. But Dombrowski reiterated that his scouts with the Detroit Tigers were also impressed with Castillo in 2014, filing reports that projected the 28-year-old as an everyday center fielder in the big leagues. Unlike Cherington, though, Dombrowski did not personally attend Castillo's workouts in Florida that left scouts so impressed with his athleticism.

Moreover, Dombrowski expressed his confidence in the Red Sox's international scouts, including senior vice president of player personnel Allard Baird, who was particularly bullish on Castillo.

"Every case is different because a lot is dependent upon how much you actually see them play and play in international tournaments," Dombrowski said. "For example, [the Tigers] probably didn't have much different scouting with Abreu. I remember we liked him when I was at Detroit, too. But in Abreu's case, it worked out, and in this case, it didn't. There's definitely more risk [in international scouting]."